I have had good results welding cast iron with two different rods- the
nickel tig rod, and silicon bronze- tig brazing the cast iron.
With the nickel, a preheat really helps, but just using the rosebud on
the oxy fuel torch will do it- 500 or 600 degrees is plenty, you dont
have to get it red.
The silly bronze rod is more forgiving, as you dont have to get the
base metal as hot.

The small Pexk Stow and Wilcox 30" shears were 18ga max. And most of
those weighed in at 400lbs. My guess is yours is a No. 30, or a No.
130- the only difference is the 130 had little fold down tables left
and right. My guess is years of people ignoring the 18ga rating, and
cutting 16ga stressed the machine out.